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Register a new userDileptons from $eta_c$ in Nucleus-Nucleus collisions
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Preliminary estimates suggest that excess dimuon production with invariant mass in the range 1.5 -- 2.5 GeV in nucleus-nucleus collisions can be explained on the basis of $eta_c$ production. This appears to be consistent with all the peripheral and central collision data with various nuclei such as S-U at 200 GeV/nucleon except for the central collision data on Pb-Pb at 158 GeV/nucleon. Some explanations based on glueball production for Pb-Pb data are discussed.
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We present an universal treatment for a substantial nuclear suppression representing a common feature of all known reactions on nuclear targets (forward production of high-pT hadrons, production of direct photons, the Drell-Yan process, heavy flavor production, etc.). Such a suppression at large Feynman xF, corresponding to region of minimal light-cone momentum fraction variable x2 in nuclei, is tempting to interpret as a manifestation of coherence or the Color Glass Condensate. We demonstrate, however, that it is actually a simple consequence of energy conservation and takes place even at low energies, where no effects of coherence are possible. We analyze this common suppression mechanism for several processes performing model predictions in the light-cone dipole approach. Our calculations agree with data.
Prompt photons produced in a hard reaction are not accompanied with any final state interaction, either energy loss or absorption. Therefore, besides the Cronin enhancement at medium transverse momenta pT and small isotopic corrections at larger pT, one should not expect any nuclear effects. However, data from PHENIX experiment exhibit a significant large-pT suppression in central d+Au and Au+Au collisions that cannot be accompanied by coherent phenomena. We demonstrate that such an unexpected result is subject to the energy sharing problem near the kinematic limit and is universally induced by multiple initial state interactions. We describe production of photons in the color dipole approach and find a good agreement with available data in p+p collisions. Besides explanation of large-pT nuclear suppression at RHIC we present for the first time predictions for expected nuclear effects also in the LHC energy range at different rapidities. We include and analyze also a contribution of gluon shadowing as a leading twist shadowing correction modifying nuclear effects at small and medium pT.
The stochastic dynamics of c and b quarks in the fireball created in nucleus-nucleus collisions at RHIC and LHC is studied employing a relativistic Langevin equation, based on a picture of multiple uncorrelated random collisions with the medium. Heavy-quark transport coefficients are evaluated within a pQCD approach, with a proper HTL resummation of medium effects for soft scatterings. The Langevin equation is embedded in a multi-step setup developed to study heavy-flavor observables in pp and AA collisions, starting from a NLO pQCD calculation of initial heavy-quark yields, complemented in the nuclear case by shadowing corrections, k_T-broadening and nuclear geometry effects. Then, only for AA collisions, the Langevin equation is solved numerically in a background medium described by relativistic hydrodynamics. Finally, the propagated heavy quarks are made hadronize and decay into electrons. Results for the nuclear modification factor R_AA of heavy-flavor hadrons and electrons from their semi-leptonic decays are provided, both for RHIC and LHC beam energies.
The propagation of the heavy quarks produced in relativistic nucleus-nucleus collisions at RHIC and LHC is studied within the framework of Langevin dynamics in the background of an expanding deconfined medium described by ideal and viscous hydrodynamics. The transport coefficients entering into the relativistic Langevin equation are evaluated by matching the hard-thermal-loop result for soft collisions with a perturbative QCD calculation for hard scatterings. The heavy-quark spectra thus obtained are employed to compute the differential cross sections, the nuclear modification factors R_AA and the elliptic flow coefficients v_2 of electrons from heavy-flavour decay.
We make a theoretical and experimental summary of the state-of-the-art status of hot and dense QCD matter studies on selected topics. We review the Beam Energy Scan program for the QCD phase diagram and present the current status of search for QCD Critical Point, particle production in high baryon density region, hypernuclei production, and global polarization effects in nucleus-nucleus collisions. The available experimental data in the strangeness sector suggests that a grand canonical approach in thermal model at high collision energy makes a transition to the canonical ensemble behavior at low energy. We further discuss future prospects of nuclear collisions to probe properties of baryon-rich matter. Creation of a quark-gluon plasma at high temperature and low baryon density has been called the Little-Bang and, analogously, a femtometer-scale explosion of baryon-rich matter at lower collision energy could be called the Femto-Nova, which may possibly sustain substantial vorticity and magnetic field for non-head-on collisions.
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